Ultraviolet Photolysis of HCHO: Absolute HCO Quantum Yields by Direct Detection of the HCO Radical Photoproduct

Absolute quantum yields for the radical (H + HCO) channel of HCHO photolysis, ΦHCO, have been measured for the tropospherically relevant range of wavelengths (λ) between 300 and 330 nm. The HCO photoproduct was directly detected by using a custom-built, combined ultra-violet (UV) absorption and cavity ring down (CRD) detection spectrometer. This instrument was previously employed for high-resolution (spectral resolution ∼0.0035 nm) measurements of absorption cross-sections of HCHO, σHCHO(λ), and relative HCO quantum yields. Absolute ΦHCO values were measured at seven wavelengths, λ = 303.70, 305.13, 308.87, 314.31, 320.67, 325.59, and 329.51 nm, using an independent calibration technique based on the simultaneous UV photolysis of HCHO and Cl2. These ΦHCO measurements display greater variability as a function of wavelength than the current NASA-JPL recommendations for ΦHCO. The absolute ΦHCO(λ) determinations and previously measured σHCHO(λ) were used to scale an extensive set of relative HCO yield measurements. The outcome of this procedure is a full suite of data for the product of the absolute radical quantum yield and HCHO absorption cross-section, ΦHCO(λ)σHCHO(λ), at wavelengths from 302.6 to 331.0 nm with a wavelength resolution of 0.005 nm. This product of photochemical parameters is combined with high-resolution solar photon flux data to calculate the integrated photolysis rate of HCHO to the radical (H + HCO) channel, J(HCO). Comparison with the latest NASA-JPL recommendations, reported at 1 nm wavelength resolution, suggests an increased J(HCO) of 25% at 0° solar zenith angle (SZA) increasing to 33% at high SZA (80°). The differences in the calculated photolysis rate compared with the current HCHO data arise, in part, from the higher wavelength resolution of the current data set and highlight the importance of using high-resolution spectroscopic techniques to achieve a complete and accurate picture of HCHO photodissociation processes. All experimental ΦHCO(λ)σHCHO(λ) data are available for the wavelength range 302.6−331.0 nm (at 294 and 245 K and under 200 Torr of N2 bath gas) as Supporting Information with wavelength resolutions of 0.005, 0.1, and 1.0 nm. Equivalent data sets of ΦH2+CO(λ)σHCHO(λ) for the molecular (H2 + CO) photofragmentation channel, produced using the measured ΦHCO(λ) σHCHO(τ) values, are also provided at 0.1 and 1.0 nm resolution.

本数据集针对甲醛（HCHO）光解的自由基（H + HCO）通道的绝对量子产率Φ_HCO，在对流层相关的300~330 nm波长范围内开展了测量。研究采用定制搭建的联用紫外（UV）吸收-腔环衰（CRD）检测光谱仪，直接探测了光解产物HCO。该仪器此前曾用于甲醛吸收截面σ_HCHO(λ)以及相对HCO量子产率的高分辨率（光谱分辨率约0.0035 nm）测量。本研究采用基于甲醛与氯气（Cl₂）同步紫外光解的独立校准技术，在7个波长点（λ=303.70、305.13、308.87、314.31、320.67、325.59及329.51 nm）测定了绝对Φ_HCO数值。相较于当前美国国家航空航天局喷气推进实验室（NASA-JPL）发布的Φ_HCO推荐值，本次测得的Φ_HCO随波长的变化幅度更大。研究将绝对Φ_HCO(λ)测定结果与此前测得的σ_HCHO(λ)结合，对大量相对HCO产率测量数据进行了定标。该处理流程得到了一套完整的数据集，涵盖302.6~331.0 nm波长范围内、分辨率为0.005 nm的自由基绝对量子产率与甲醛吸收截面的乘积Φ_HCO(λ)σ_HCHO(λ)。将上述光化学参数乘积与高分辨率太阳光子通量数据结合，可计算得到甲醛经由（H + HCO）自由基通道的积分光解速率J(HCO)。与以1 nm波长分辨率发布的最新NASA-JPL推荐值对比发现，在太阳天顶角（SZA）为0°时，J(HCO)提升了25%，在高SZA（80°）下提升幅度增至33%。本次计算得到的光解速率与现有HCHO数据存在差异，部分原因在于本数据集具备更高的波长分辨率，这也凸显了采用高分辨率光谱技术以完整准确地刻画甲醛光解离过程的重要性。所有实验测得的Φ_HCO(λ)σ_HCHO(λ)数据以补充材料形式发布，覆盖302.6~331.0 nm波长范围（实验条件为294 K、245 K以及200托（Torr）氮气（N₂）载气），并提供了0.005、0.1及1.0 nm三种波长分辨率的数据集。本数据集还提供了分辨率为0.1 nm与1.0 nm的分子（H₂ + CO）光解通道对应的等效数据集Φ_H₂+CO(λ)σ_HCHO(λ)，该数据集基于本次测得的Φ_HCO(λ)σ_HCHO(τ)数值生成。